This invention relates to a method and apparatus for structurally enhancing pipe sections and facilitating their handling and, more particularly, this invention relates to a method and apparatus to facilitate the installation, operation and maintenance of deepwater offshore pipelines.
Effective installation and operation of deepwater subsea pipelines are increasingly necessary as the need for new sources of oil and gas push development into deeper water offshore. These deepwater applications present several problems to the installation and operation of pipelines which are not significantly encountered in the more shallow developments of the past.
First, pipe laying operations are complicated with the increased water depth. Pipeline is laid from a barge, work boat or other especially equipped vessel to add new sections of pipe, one at a time, to the end of the pipeline, lowering the pipeline to the seafloor and advancing the vessel as new sections are added. The "S" lay technique in which additional sections are joined to the pipeline on a horizontal work deck, axially rolled down a shallow angle stinger and settled to the ocean floor in a long S-shaped curve, is not appropriate for deeper water.
The deepwater applications are best served by a vertical or near vertical joining operation which feeds additional pipe sections which leaves the pipelay vessel in a substantially vertical orientation, carrying this orientation down to proximity with the ocean floor where it curves to follow the contours of the seafloor.
One problem is that the end of the pipeline must be secured at the pipelay vessel in its substantially vertical orientation against the vertical load exerted by the weight of the pipeline. Before each new section of pipe is added, the last previous section joined is allowed to slip downward, below the pipelay vessel, until the top of the pipeline is in position for adding the next section. Then, it must be secured and any slip may cause the end of the pipeline to advance below the securing facilities of the vessel, thereby losing the end of the pipe which then falls to the ocean floor. This is likely to severely damage the pipeline and any recovery, if possible at all, would prove expensive, difficult and time consuming, particularly under the prior art. Thus, secure retention of the upper end of the pipeline is a critical issue to deepwater pipelaying operations. However, present methods of addressing this are inadequate to meet the needs in deepwater applications. For instance, tensioners which are used to advance the pipeline operate by securing the pipeline with skin friction and are poorly suited for use in vertical oriented pipelines. The free end of the tensioner is exposed at too high an elevation for convenient joining operations due to the length of the tensioner necessary to achieve sufficient skin friction to secure the pipe.
Alternatively, clamps which tightly "bite" into the surface of the pipe are inappropriate because such operations destroy at least the protective coating on the pipe, and may structurally damage the pipeline directly. In fact, present handling techniques are not even well suited to vertically orienting individual sections of pipe for joining to the vertical end of the pipeline.
Another problem of laying pipeline in deep water is the danger of buckling the pipe and the likelihood of propagating this failure, once initiated, along the length of pipeline. Damage to the pipeline during installation or later from falling debris can leave stress concentrations susceptible to buckling failure. Even the suspension of the pipeline across bottom irregularities such as ridges or valleys on the ocean floor can produce additional loads on the pipeline rendering it susceptible to buckling, particularly if there is a pressure loss within a high pressure gas pipeline designed in light of an internal pressure to offset some portion of the intense pressure of the ocean depths. Once buckling has initiated, the edge of the buckled region becomes a moving wave of stress concentration as the edge of the buckled, flattened region goes out of round and becomes less efficient in distributing the force of the great water pressure and the resistance of the pipeline plummets once the yield point has been crossed.
Finally, when there is any accident or damage to the pipeline it is very difficult to retrieve any portion of the pipeline to the surface for repairs. Repairs are very difficult to effect at the ocean floor in deepwater tracks of the pipeline.